The invention relates to a charge coupled device (CCD) sensitive to infrared radiation in a range lying between the wavelengths .lambda..sub.0 and .lambda..sub.1. The CCD comprises a succession of three layers of a semiconductor material consisting of elements from Groups III and V of the periodic table of the elements. The layers have crystal lattice parameters which are extremely close to each other.
The first layer of the CCD is designated as the window. The window has a large forbidden energy band so that it does not significantly absorb radiation having wavelengths greater than .lambda..sub.0. The second so-called absorbing layer has a narrower forbidden energy band so that it absorbs radiation having a wavelength up to .lambda..sub.1. The third so-called storage layer has a large forbidden energy band (i.e. a small wavelength absorption limit). At least one electrode is in ohmic contact with the storage layer and forms the output O of the device. A plurality of control electrodes are also periodically arranged on the surface of the third (storage) layer so as to form a line or a matrix of photo-sensitive elements. The electrodes are insulated from each other and are sequentially polarized by the signals of one or several clock generators H.sub.1, H.sub.2 . . . H.sub.n.
The invention further relates to a method of manufacturing this device.
Such a CCD is described in an article by Y. Z. Liu et al entitled "Observation of charge storage and charge transfer in a GaAlAsSb/GaSb charge-coupled device", Applied Physics Letters, Vol. 36, No. 5, pages 458-461, Mar. 14, 1980.
This document discloses a device sensitive to infrared radiation in the range lying between 1.0 .mu.m and 1.8 .mu.m. Such a device is of importance for military terrestrial applications (for example for use in infrared imaging devices), and for submarine applications.
In the wavelength range of from 0.9 to 2 .mu.m, ambient radiation is considerably stronger than in the wavelength range below 0.9 .mu.m. Thus, from about 1 .mu.m, the contribution of photons thermally radiated by an object to be detected is very substantial. This phenomenon permits very satisfactory imaging of such objects, if devices sensitive to these wavelengths are available.
The Liu et al article discloses a device having a window layer of GaAlAsSb. GaAlAsSb is a quaternary compound having a large forbidden energy band (i.e. an absorption limit on the order of 1 .mu.m). The device has a sensitive layer of GaSb, a binary compound having a narrow energy band (i.e. and absorption limit on the order of 1.8 .mu.m). Finally, the device has a storage layer of GaAlAsSb, a quaternary compound having a large forbidden energy band (i.e. a small absorption limit). The control electrodes form Schottky barriers with the storage layer.
A method of manufacturing this CCD starts with a GaSb substrate. The first, second and third layers are grown in the substance by epitaxy from the liquid phase. This assembly can be fused to a glass plate for mechanical support, for protection for the rear (window) layer, and for filtering small wavelengths.
Subsequently, the starting substrate of GaSb can be chemically removed, and the electrodes can be provided by conventional methods.
In this method, the starting GaSb substrate can be produced only with great difficulty. The substrates obtained by known techniques contain numerous dislocations. It is clear that the epitaxial layers formed on such substrates also contain large numbers of dislocations.
Moreover, the GaSb substrates obtained have very small diameters. They are not suitable for obtaining a large number of CCD's on the same wafer or even for obtaining CCD's of large surface areas. This is a great disadvantage of the method with respect to the short term industrial future.
Furthermore, since the demixing range of the compound is very large, the quaternary layer of (Ga,Al)(As,Sb) can be processed only with very great difficulty.
Moreover, the use of a quaternary layer as a storage layer makes manufacturing the various electrodes very difficult. Therefore, the control electrodes must be electrodes which form Schottky barriers. The ohmic contacts are also difficult to obtain. Finally, it is necessary to form a supplementary ohmic contact for the guard anode. The guard anode is a gate which is polarized so that it depletes the n-type storage layer between the control electrodes.
Finally, this device must be cooled to 77.degree. K. so as to reduce the dark current.
However, the most serious disadvantage is that no synergy at all exists between the manufacture of such a CCD and the manufacture of integrated circuits to be used with the CCD. This is certainly a problem as to the industrial development of this novel technology because it is very expensive to utilize a difficult technology to obtain only a small number of devices. However, this is an even larger problem for the manufacture of the CCD itself.
In fact, it is necessary to add peripheral circuits, such as an output amplifier and a clock signal generator, which are ultrahigh speed or superhigh frequency devices, to the CCD. It is desirable to be able to integrate these circuits monolithically on the same substrate as the CCD so as to increase the speed and the reliability of the assembly, while reducing its manufacturing cost and the technological difficulties in the manufacture of the CCD. This proves to be impossible by means of the technology described by Liu et al.